APRIL FOOLS is somewhat of an April Fools' Day bait-and-switch video, made by SuperMarioGlitchy4.
Mario is apparently filming an exercise video in the castle lobby: he opens the blooper asking the viewers if they are ready to Mario-cise. Just as he is about to begin the workout, however, SMG4 runs on-screen and punches Mario to the ground with a punch. Large text is edited in, reading "MISSION ACCOMPLISHED!"; at the same time, loud music and an explosion from out of the blue prompt several random characters to appear and start dancing.